The findings also take into account that one's chronological age may differ from one's biological age.
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A new saliva test developed by geneticists at the University of California, Los Angeles, reveals a person's age within five years, a finding that could have many applications in medicine, at crime scenes, and more.

"With just a saliva sample, we can accurately predict a person's age without knowing anything else about them," says principal investigator Dr. Eric Vilain, a professor of human genetics, pediatrics and urology, in a UCLA news release.

The team's research, published online this week in the Public Library of Science's PLoS One journal, focuses on methylation, a process by which chemicals called methyl groups bind to DNA, guiding which genes become active. Because methylation patterns change with age, the team hoped that analyzing genes from a saliva sample might indicate one's actual age.

After analyzing saliva samples from 34 pairs of identical male twins aged 21 to 55, the team sifted through their genomes (identical twins having near-identical genes) and was able to identify 88 sites on the DNA that suggest a high correlation between methylation and age.

Dr. Eric Vilain
UCLA

The team then performed the study again in a different population--a random group of 61 men and women aged 18 to 70. This time they looked at just two of the three genes with the strongest age link to methylation, created a predictive model, and analyzed both groups' saliva samples.

Using this predictive model, the team accurately predicted each person's age within five years, which they call an unprecedented level of accuracy.

"Methylation's relationship with age is so strong that we can identify how old someone is by examining just two of the 3 billion building blocks that make up our genome," says first author Sven Bocklandt, a former UCLA geneticist who now works at Bioline.

The researchers have just patented a test based on their findings, but they aren't stopping there. Because methylation does not correlate exactly with chronological age (as measured by calendar years) in some humans, they want to study this population further to develop a way to test a person's biological age.

Which is where the research gets really interesting. Because people age at different paces due to a unique combination of genes, environment, and lifestyle, some have a "low" bio age (with, say, 60 calendar years behind them but the biological body of an average 50-year-old), while others have a "high" one (with, say, 60 calendar years behind them but the biological body of a 70-year-old).

We use calendar age because it is constant and largely reliable, but using biological age in some patients could help physicians change their evaluation of age-related diseases in routine screenings--suggesting, for instance, colonoscopies earlier or later depending on the person's bio age.

"Doctors could predict your medical risk for a particular disease and customize treatment based on your DNA's true biological age," as opposed to how many years you've been alive, Vilain says. "By eliminating costly and unnecessary tests, we could target those patients who really need them."

The UCLA team next hopes to determine whether people with a lower bio age live longer and whether a higher bio age is linked to a greater rate of disease and shorter life span.